1. Field of the Invention
The invention generally relates to compact electronic display systems. More specifically, the invention relates to illumination systems for use with compact electronic display systems.
2. Description of Related Art
A continuing objective in the field of electronics is the miniaturization of electronic devices. Most electronic devices include some form of display system which provides information to the user. As electronic devices are miniaturized, display systems are needed which can be incorporated into increasingly smaller devices. It is thus important that the space required to house these display systems be reduced. In one regard, it is desirable that the thickness of the display system be reduced, the thickness of the display referring to the dimension of the display system which is perpendicular to the plane of the image formed by the display.
In general, the image provided by an electronic display may be either a real image or a virtual image. One approach to reducing the size of a display system is through the formation of a virtual image instead of a real image. A variety of virtual image displays have been described including those described in U.S. Pat. Nos. 5,644,323; 5,625,372; 5,684,497; 5,771,124; 5,838,498; 5,870,068; 5,892,624; 5,905,478, each of which are incorporated herein by reference.
A real image refers to an image which is observed directly by the unaided human eye. A real image exists at a given location when a real image can be observed by the unaided eye if a viewing surface is positioned at the location. A photograph is an example of a real image. Examples of electronic displays which provide real images include liquid crystal displays, CRT monitors, and projection screens. Compact electronic devices because of their small size, have a limited surface area on which to provide a real image. Since the amount of detail that the human eye can resolve per unit area is limited, devices which provide a real image are only able to provide a limited amount of legible information per display screen.
By contrast to real image displays, virtual image displays provide a virtual image, i.e., an image which, if a viewing surface were positioned at the location of the virtual image, no image would be observed by the eye. By definition, a virtual image can exist at a location where no display surface exists. An example of a virtual image is the image of fine print viewed through a magnifying glass.
In the case of a virtual display, the image is first formed by an imaging component referred to as the source object. As with real image displays, the source object may be a CRT or a liquid crystal display, but it is usually miniaturized to reduce the overall size of the display system. Magnifying optics are then utilized to form the virtual image that is actually seen by the viewer. Source objects may actually generate the light that is viewed as in the case of a CRT or luminescent device. Such a display is said to be an emissive display. Alternatively, the display may be non-emissive such as a liquid crystal display which requires illumination from an external source of light.
Virtual image displays provide an image which appears to be larger than the source object. As a result, the size of the virtual image, as perceived by the user, is limited by the magnification of the display system as opposed to the size of the electronic display. This enables virtual image displays to provide the user with a greater amount of legible information per display screen than real image displays in the same space. It also enables a virtual image display to be designed which provides the same amount of information per screen as real image displays in a smaller space.
An important aspect of the functioning of a virtual image display is the ease of viewing the virtual image. In this regard, it is important for the display to provide a wide apparent angular width of the virtual image, commonly referred to as the field of view of the virtual image. The full field of view is defined as the ratio of the largest apparent dimension of the virtual image to the apparent distance to the virtual image. It is generally equivalent to the field of view for a real image display surface. A need exists for a virtual image display system which provides a wide field of view.
A wide field of view requires a large aperture through which the image is viewed in order to have a comfortable distance between the display and the eye, which is referred to as the eye relief of the display. Another important property of a virtual display is a large amount of space within which the eye can move and still see the image. This space is referred to as the eye-box.
If the source object of the display is non-emissive, the display must be illuminated by a source of light such as a lamp or LED. When the microdisplay is a non-emissive, reflective display, the illumination must be provided from the front side of the display, which is made more difficult by the need to view the image created on the front side of the display. It is important that this illumination provide ample light such that the resulting image is bright and that the image has sufficient contrast between bright and dark portions of the image. In the case of a transmissive liquid crystal display, the illumination is provided from the rear side of the display and is referred to as backlighting.
Since virtual image displays are frequently used in portable devices, it is important for the displays to be energy efficient. Hence, it is desirable for the displays to use a low amount of energy for illumination. A need therefore exists for a display system which provides a bright virtual image. A further difficulty associated with virtual image displays is irregularities in the illumination of the source object. A need therefore also exists for a display system which provides a virtual image having substantially uniform illumination across the image.
The present invention relates to illumination systems for use in virtual image display systems. In general, the virtual image display systems of the present invention include a non-emissive, reflective microdisplay which forms a source object; an optical system which forms a magnified, virtual image of the source object from light reflected off the microdisplay; a light source system which produces light to illuminate the microdisplay; and illumination system according to the present invention.
In one embodiment, the illumination system forms at least two virtual light sources to illuminate the microdisplay. In one variation, the at least two virtual light sources include a first virtual light source positioned approximately in front of and directly over the microdisplay and second virtual light source positioned over and to one side of the microdisplay. The illumination system may also form three or more virtual light sources to illuminate the display system. In one variation, the at least three virtual light sources include a first virtual light source positioned approximately in front of and directly over the microdisplay, a second virtual light source positioned over and to a first side of the microdisplay, and a third virtual light source positioned over and to a second side of the microdisplay.
In another embodiment, the illumination system directs light from the light source system through the illumination system to the microdisplay such that the angles of incidence of light which illuminates the microdisplay varies across the microdisplay over an angular range that is greater than would be provided by the light source system if the light source system were to directly illuminate the microdisplay.
In another embodiment, the illumination system directs light from the light source system through the illumination system to the microdisplay such that the angles of incidence of light which illuminates the microdisplay varies across the microdisplay over an angular range of at least 40 degrees. In one variation, the angles of incidence of light which illuminates the microdisplay vary over an angular range of at least 60 degrees. In another variation, the angles of incidence of light which illuminates the microdisplay vary over an angular range of between about 40 degrees and 120 degrees, more preferably between about 60 degrees and 100 degrees.
In another embodiment, the illumination system directs light from the light source system through the illumination system to the microdisplay such that a first portion of the light incident on the microdisplay is approximately perpendicular to the microdisplay, a second portion of the light incident on the microdisplay is positively angularly displaced relative to the perpendicular to the microdisplay, and a third portion of the light incident on the microdisplay is negatively angularly displaced relative to the perpendicular to the microdisplay.
In another embodiment, the illumination system includes an illumination body which transmits light from the light source system to the microdisplay to illuminate the microdisplay where a first potion of the light is internally reflected off one surface of the illumination body prior to illuminating the display system and a second portion of the light is internally reflected off two surfaces of the illumination body prior to illuminating the display system. According to this embodiment, at least one of the internal reflections may be a total internal reflection. According to this embodiment, multiple and optionally all of the internal reflections may be total internal reflections. Also according to this embodiment, the first portion of light from the light source may traverse the illumination body by being internally reflected off of a surface of the illumination body adjacent the optical system prior to reaching the microdisplay. Also according to this embodiment, the second portion of light from the light source may traverse the illumination body by being internally reflected off of a surface of the illumination body adjacent the microdisplay, then internally reflected off of a surface of the illumination body adjacent the optical system prior to reaching the microdisplay. Also according to this embodiment, a third portion of the light may be internally reflected off the surfaces of the illumination body three times prior to illuminating the display system.
In another embodiment, the illumination system includes an illumination body which transmits light from the light source system to the microdisplay to illuminate the microdisplay, the light source system being positioned adjacent a first surface of the illumination body, the microdisplay being positioned adjacent a second surface of the illumination body, and the optical system being positioned adjacent a third surface of the illumination body, where a portion of the light from the light source traverses the illumination body by being internally reflected off the third surface of the illumination body. According to this embodiment, at least one of the internal reflections may be a total internal reflection. Optionally all of the internal reflections may be total internal reflections.
Also according to this embodiment, a portion of the light from the light source may traverses the illumination body by being internally reflected off the second surface of the illumination body and then internally reflected off the third surface of the illumination body. Also according to this embodiment, a first portion of the light from the light source traverses the illumination body by being internally reflected off the third surface of the illumination body, and a second portion of the light from the light source traverses the illumination body by being internally reflected off the second surface of the illumination body and then internally reflected off the third surface of the illumination body. Also according to this embodiment a first portion of the light from the light source traverses the illumination body by being internally reflected off the third surface of the illumination body, a second portion of the light from the light source traverses the illumination body by being internally reflected off the second surface of the illumination body and then internally reflected off the third surface of the illumination body, and a third portion of the light from the light source traverses the illumination body by being internally reflected off the third surface of the illumination body, then internally reflected off the second surface of the illumination body, and then internally reflected off the third surface of the illumination body.
In regard to any of the above embodiments, the illumination system may comprise an illumination body which forms the at least two virtual light sources. The illumination body may include a prism which functions to form the at least two virtual light sources. The light source system may be positioned adjacent a first surface of the prism, the microdisplay being positioned adjacent a second surface of the prism, and the optical system being positioned adjacent a third surface of the prism. The light source system produces polarized light and the third prism surface is attached to a reflective polarizer. In one variation, the angle between the third prism surface and the second prism surface is less than 45 degrees, more preferably between 20 and 35 degrees. In a further variation, the angle between the first prism surface and the second prism surface is between about 1.7 and 2.3 times the size of the angle between the third prism surface and the second prism surface.
The present invention also relates to light source systems for use in virtual image display systems. In general, the virtual image display systems include a non-emissive, reflective microdisplay which forms a source object; an optical system which forms a magnified, virtual image of the source object from light reflected off the microdisplay; a light source system according to the present invention which produces light to illuminate the display system; and an illumination system which directs light from the light source system to the microdisplay. It is noted that the illumination system may be an illumination system as taught herein or may be a different illumination system.
An embodiment of a light source system according to the present invention comprises a light source body which includes a light entry surface across which light from a light source element enters the light source system, a light exit surface across which light is transmitted from the light source system to the illumination system, and a first diffusing/reflective region positioned opposite the light exit surface which includes an internally reflective surface and a diffusing region internal to the internally reflective surface, the diffusing region diffusing light which traverses the diffusing region.
According to this embodiment, all of the surfaces of the light source body other than the light entry surface and the light exit surface are preferably internally reflective and more preferably highly diffusely reflective.
In one variation, the light entry surface is orthogonal to the light exit surface. In this regard, light which enters through the light entry surface is first internally reflected before reaching and exiting the light exit surface. In this regard, the internally reflective surface of the first diffusing/reflective region and the light exit surface are preferably angled relative to each other so as to direct light to the light exit surface. The angle between the internally reflective surface of the first diffusing/reflective region and the light exit surface is preferably between about 10 degrees and 45 degrees, more preferably between about 10 degrees and 25 degrees, most preferably between about 11 degrees and 13 degrees. The internally reflective surface of the first diffusing/reflective region preferably has a length of at least 10-mm, more preferably between 10-14-mm. The light exit surface preferably has a length of at least 10-mm, more preferably between 10-12-mm.
According to any of the above variations, the light source system may further include one or more optical layers positioned between external to the light exit surface, the one or more optical layers performing one or more functions selected from the group consisting of columniation and polarization. In one variation, the one or more optical layers includes a plurality of optical layers where an air gap is positioned between adjacent optical layers.
The optical layers may include a first and a second collimators positioned external to the light exit surface. An air gap is preferably positioned between the first and second collimators. The first and second collimators are preferably at right angles relative to each other. A polarizer may also be positioned external to the light exit surface adjacent the collimators.